1. Field of the Invention
This invention relates to novel liquid-crystalline compounds useful as electrooptical display elements.
2. Description of the Prior Art
Liquid crystals which have currently been broadly used as display elements are nematic liquid crystals. When the methods of display are roughly classified, TN type (twisted nematic type), DS type (dynamic scattering type), DAP type (electric field-controlling birefringence type), PC type (cholesteric-nematic phase-transition type), GH type using dichromatic dye (guest-host type), etc. are representative. Smectic liquid crystals have so far been not broadly employed for practical uses, but it is possible to add them to nematic liquid crystals depending on their compatibility with nematic liquid crystals, to thereby improve their characteristic properties, and further, in recent years, since it has become possible to display large capacity informations by way of a matrix display utilizing a electrothermo-optic effect, their practical use has been hurried up. Furthermore a display method according to which the response speed is higher than that of TN type has been actively developed. As to liquid-crystalline materials currently used for these dispaly elements, none of single compounds cannot endure practical use as regards their various characteristic properties i.e. liquid-crystalline temperature range, actuation voltage, response performance, etc.; thus it is the present status that compounds which endure practical use have been obtained by blending several kinds or in some case, about 20 kinds of liquid-crystalline compounds.
In particular, liquid-crystalline display elements have recently been also used for cars, etc., and those which can be used within a broader temperature range and also have a higher response speed have been needed. In order that such liquid-crystalline compositions are composed, higher temperature liquid-crystalline compounds (i.e. those of which the liquid-crystalline temperature range has a higher upper limit) and lower viscosity liquid-crystalline compounds are indispensable as their components. Usually, the former higher temperature liquid-crystalline compounds have intrisically a higher viscosity, while the latter lower viscosity liquid-crystalline compounds such as those expressed by the formulas ##STR4## have no satisfactorily low viscosity, and also they usually have their liquid-crystalline temperature range on the lower side of such ranges to function so as to reduce the liquid-crystalline temperature range of the whole of the composition whereby their amount added is restricted. Thus, compounds having a lower viscosity and also a liquid-crystalline temperature range as high as possible have been desired.
The object of the present invention is to provide compounds which are useful as a lower viscosity liquid-crystalline component of which such liquid-crystalline compositions are partly composed.